1. Field of the Invention
This invention relates to an image forming apparatus such as an electrophotographic copier or printer for forming an image by sequentially forming toner images of different colors on a photoreceptor drum and sequentially transferring, in superimposition, the toner images from the photoreceptor drum onto a transfer material carried by a transfer drum and, in particular, to a technique for attaching a transfer film that is a part of the transfer drum.
2. Description of the Related Art
For example, such a conventional image forming apparatus using a transfer drum forms an image by sequentially forming toner images of different colors such as cyan, magenta, yellow, and black on a photoreceptor drum and sequentially transferring, in superposition, the toner images from the photoreceptor drum to a transfer material such as a transfer sheet carried by a transfer dry. After the transfer of a given number of multiple toner images onto the transfer material that is adsorbed on the transfer dry, the transfer material is stripped off the surface of the transfer drum and the toner images are fused on the transfer material by a fuser.
For example, as shown in FIG. 14, a transfer drum 100 comprises a drum-like free 103 made of metal, for instance, and having a pair of ring members 101 at both ends in the axial direction and a tie-bar plate 102 for coupling the ring members 101 to each other. A circumferential front end portion of a transfer film 104 made of a dielectric film of polyethylene terephthalate, polyvinylidene fluoride, or the like is fixed to the tie-bar plate 102; both side end portions of the transfer film 104 are wound around outer peripheral surfaces of the respective ring members 101 so as to conform to the letter; and a circumferential rear end portion of the transfer film 104 is fixed to the tie-bar plate 102. Thus, the transfer film 104 assumes a hollow cylinder. The front end portion of the transfer film 104 is fixed to the tie-bar plate 102 with screws 105.
The rear end portion of the transfer film 104 is fixed to the tie-bar plate 102 as follows. As shown in FIG. 15B, a rear end plate 107 is fixed to the bottom face of the rear end portion of the transfer film 104 by, for instance, bonding via a urethane sponge 106. As shown in FIG. 15A, the transfer film 104 and the urethane sponge 106 are formed with substantially U-shaped relief portions 108, and insertion holes 109 for screwing are formed in the rear end plate 107 at positions corresponding to the relief portions 108. The rear end portion of the transfer film 104 is fixed to the tie-bar plate 102 by inserting screws 105 into the insertion holes 109 of the rear end plate 107 and threadedly engaging the screws 105 to female screws formed in the tie-bar plate 102.
However, the above conventional image forming apparatus has the following problem. As shown in FIG. 14, when it is attempted to attach the transfer film 104 to the frame 103 of the transfer drum 100, if the length, etc., of the transfer film 104 varies due to the working accuracy of the transfer film 104 itself, variations in the attachment positions of the rear end plate 107 to the transfer film 104, or changes in environmental conditions such as the temperature and humidity, there may occur a case that the positions of the insertion holes 109 of the rear end plate 107 do not match those of the female screws of the tie-bar plate 102 and the rear end plate 107 cannot be screwed to the tie-bar plate 102. In this state, if an attempt is made to forcibly secure the rear end plate 107 of the transfer film 104 to the tie-bar plate 102 by screws, the rear end portion of the transfer film 104 is much deformed and a transfer material held on the transfer film 104 is deformed together (see FIGS. 16 and 17), thus causing image defects in toner images transferred onto the transfer material.
A technique that can solve the problem is already proposed in Japanese Patent Laid-Open No. Hei. 4-274269. An image forming apparatus according to this publication forms developed images on an image carrying body and transfers the developed images to a transfer material carried by a transfer material holding member of a transfer device, to form an image. The transfer device comprises a transfer drum frame having a pair of annular rotary members and a coupling member for coupling the annular rotary members. The coupling member is provided with retention members for retaining the transfer material holding member, and an elastic member having U-shaped grooves to engage the necks of the respective retention members is formed integrally with a front end portion of the transfer material holding member. The transfer material holding member is made longer than the outer perimeter of the transfer drum frame, and a rear end portion of the transfer material holding member overlapping with the tip of the transfer material holding member is fixed to the front end portion of the transfer material holding member. For example, the rear end portion of the transfer material holding member is bonded to the front end portion of the transfer material holding member.
In this case, however, since the rear end portion of the transfer material holding member is fixed to the front end portion of the transfer material holding member by, for instance, bonding, the tolerable range of deformation of the transfer film with respect to environmental variations is small. Therefore, under a low-temperature condition, the transfer film shrinks to form a recess, causing a transfer failure. Under a high-temperature condition, it expands and becomes loose, creating a color shift, etc., resulting in an image defect.